A N A L Y T I C A L CHEM1 STRY
374 Table 11. Maximum Quantities of Cations Foreign Cation Highest Quantity Minimum Quantity Present in of Forei n of Copper Ion Copper Solution Cation Use8 per Detectable per to Be Tested Macrodrop Macrodrop 0.2 Au (111) ' 500 0.2 Bi(II1) 500a 1 F e (111) 200 0.2 Ag (1) 2500 a Maximum quantity permissible.
Table 111. Anion Present in Copper Solution
1:2500 1: 2500
1:200 1 : 12,500
Sensitivity of Test
Quantitya of Anion per Macrodrop
Minimum Quantity of Copper Ion Detectable per hlaorodrop
Y
0
Ratio of Copper Ion to Foreign Cation
Y
Nitrate I550 2090 Chlorate 3200 Bromate Iodate 4380 4800 Periodate These quantities correspond to solutions t o 0.5 mole.
0.1
0.5 0.5 0.5 0.5
brownish coloration of the center, surrounded by the violet ring. Thus, 1.0 microgram of copper can be detected in the presence of 200 micrograms of iron. 3. In the presence of silver compounds one drop of the solution to be tested is boiled for a short while with one drop of reagent 1. Silver bromide precipitates, and one drop of the liquid is taken for the ordinary test. This reaction allows detection of 0.2 microgram of copper in presence of 2500 micrograms of silver. Upon exposure to light, the silver bromide which collects in the center of the spot immediately turns gray. 4. If such colored substances as Cr (111), Ni (II), and Co (11) compounds are present, the test is applicable only if their concentration is not higker than that indicated in Table I. If large quantities are present, they must be removed from the solution by one of the ordinary analytical methods. Table I1 indicates the maximum quantities of foreign cations per macrodrop n-hich will not interfere Kith the modified specific procedures.
West (8) observed that nitrates, chlorates, bromates, iodates, and periodates give an intense yellow stain under the coqditions of this reaction. The color is obviously due to free bromine liberated by these highly oxidizing substances. In the case of nitrates present in the solution, the test is not affected adversely, as the violet ring is formed before the yellow color appears. In the presence of chlorates and bromates the violet ring appears first, but is immediately obscured by a deep yellow stain. Fortunately, the color fades quickly, and after renewed drying, only the violet ring, characteristic of copper, reappears. The situation is somewhat more. complicated in the case of iodates and periodates, which give a very persistent yellow stain. In this case, the reaction is performed in the following manner, utilizing the difference in capillarity of copper ions and these anions: One drop of the solution to be tested is dried on the paper, and one drop of reagent 1 is added and dried. Another two drops of reagent 1 are then brought onto the yellow spot. Upon drying, a yellow ring appears and, after some time, an outer violet ring. The authors have determined the sensitivity of the method in the presence of the oxidizing anions; their results are summarized in Table 111. LITERATURE CITED
Augusti, S.,Jfikrochemie, 22,139 (1937). Carter, S.R., and Megson, N. J. L., J . Chem. SOC.,1928,2954. Cresti, L., Ber., 10, 1099 (18977). Feigl, F., "Qualitative ..inalysis by Spot Tests," 3rd English ed., p. 74, New York, Elsevier Publishing Co., 1946. Kirchhof, Kautschuk, 14,163 (1938). Sabatier, P., Compt. rend., 118,980 (1894); quoted by GnielinKraut, 7th ed., Vol. V, Part I, pp. 708, 938. Szheringa, K., Chem. Zentr., 1925, I, 1771; Pharna. Weekblad, 62, 173 (1925). West, P. TV., private communication. RECEIVED September 30,
1946.
Melting-Point Bath liquids JONATHAN L. HARTWELL, .Vational Cancer Institute, National Institute of Health, C'. S . Public Health Service, Bethesda, M d .
HE paucity of published material relating to substitutes for Tconcentrated sulfuric acid as the bath liquid in the common melting-point apparatus is difficult to account for in view of the disadvantages of this almost universally used liquid, chief of which are its hygroscopicity and its hazards. I n particular, it is annoying and time-consuming when a melting-point determination must be interrupted because absorption of water vapor has lowered the boiling point of the bath below its expected value, and it is dangerous when one is working with unstable compounds or with compounds that melt near the boiling point of the acid. Yet the advantages of the type of apparatus employing a liquid heat-transfer medium, especially the Hershberg apparatus 1 2 ) for the accurate determination of corrected melting points by total thermometer immersion, seemed so great that a search was made for a substance that would be generally superior t o sulfuric acid. The classic treatises on organic chemical methods suggest only phosphoric acid, paraffin oil, paraffin, glycerol, castor oil, rapeseed oil, mixtures of sulfuric acid with inorganic sulfates, and mixtures of inorganic salts. Some of these have the same disadvantages as sulfuric acid; others are not liquid a t room tempera-
ture. More recently there have been recommended HB-40 (a), a product consisting mainly of a mixture of terphenyls, and silicone fluid type 550 (4, an aromatic silicone. After over 60 different organic substances distributed among several types Thich seemed to offer promise, had been investigated, two were ultimately selected as superior to the others and to those described in the literature--Aroclor 1248 and silicone oil 9981-LTNV-40. Both products are liquid a t room temperature, boil above 350" C. (corrected), are colorless, noncorrosive, highly stable to heat, nonhygroscopic, without objectionable odor a t elevated temperature, have a high flash point, and are commercially available in large quantities. The former gives tovic effects only upon prolonged inhalation of the vapor, n-liile the latter is nontoxic. EXPERIMEATAL
Preliminary Screening. A large number of substariccs w r c heated to boiling in an open beaker with a thermometer immersed in the liquid. Of these, 62 registered a temperature of 298O C. (corrected) or above. Some n-ere solids, as it was thought pos-
,
V O L U M E 20, NO. 4, A P R I L 1 9 4 8
375
sible that a mixture of a solid compound with Table 11. Substances Rejected o n Further Screening just enough liquid compound to render the mixture liquid at room temperature might Substance Supplier Description yield the best result. Hydrocarbons n-Octadecane Connecticut Hard Rubber Co. Of the 62, 24 were colored, usually yelEmbedding paraffin R. F. Revson Co. Technical ceresin Eimer and Amend low, and 38 vere either colorless or only Santowax M H Monsanto Chemical Co. Hydrogenated m-terphenyl yellow tinted. The substances which ppsHB-40 Monsanto Chemical C o . Mainly a mixture of terphenyls sessed color (Table I) were set aside temAlcohols porarily and, when it was found that the Cetyl alcohol Eastman Kodak Co. Heptadecanol Carbide and Carbon Chemicolorless substances were yielding the desired cals Corp. * Ethers results, nothing more was done with them. Di-n-decyl ether Connecticut Hard Rubber Co. Di-n-dodecyl ether Further Screening. The 36 compounds that Connecticut Hard Rubber Co. Esters were colorless or only yellow tinted before Cetyl acetate Eastman Kodak Co. Eastman Kodah Co. n-Octadecyl acetate heating, with S o s . 6 and 9 (Table 111) which Arnold, Hoffman and c o . Butyl stearate were light yellow in color but were studied Eastman Kodak Co. n-Butyl sebacate Distillation Products, Inc. n-Amyl sebacate Amoil-S when it was found that silicones were so Distillation Products, Inc. 2-Ethylhexyl sebacate Octoil-S Eastman Kodak Co. n-Butyl phthalate stable to heat, !=,-ere then subjected to a proZEthylhexyl phthalate Distillation Products, Inc. Octoil longed heat-stability test designed to acceler2-Ethylhexyl phthalate Carbide and Carbon ChemiFlexol plasticizer D O P cals Corp. ate the production of color or other effects proButyl phthalyl butyl Santicizer B-16 Monsanto Chemical Clo. collate duced on continuous use in the melting-point Santicizer R-15 Monsanto Chemical Co. E t h y l phthalyl ethyl glyapparatus (1). Samples were placed in test collate Monsanto Chemical Co. hfethyl phthalyl ethyl glySanticizer M-17 tubes which dipped into an electrically heated , collate E t h e r esters Wood's metal bath. I n one tube was placed Arnold, Hoffman and Co. Butyl Cellosolve stearate a standardized Anschutz thermometer, the Sharples Chemicals, Inc. p-tsrt-.4mylphenoxyet hpl laurate mercury column was totally immersed, and I f e t a 1 organics Eastman Kodak Co. Tri-o-cresyl phosphate all tubes were lightly stoppbred. The liquids Eastman Kodak Co. Tri-m-cresyl phosphate were first brought to 200" (corrected). Eastman Kodak Co. Tri-p-cresyl phosphate Tricresyl phosphate Kavalco Products Iironitex -4 Within 24 hours, 27 substances had become yellow t o dark brown and were rejected (Table 11). The eleven remaining, which did not darken DISCUSSION in color at 200' C. (corrected), were left in the JT'ood's metal bath From Table I11 it is clear that, after sulfuric acid has been reand brought t o 250" (corrected). After 48 hours the temperajectedbecause of its unsuitability for use a t 250" C.,the compounds ture was increased to 300' (corrected) and maintained there for that stand out for their heat stability are the Aroclors (Sos. 1, 2, 48 hours. The results with these compounds, with concentrated and 3) and the silicones (Nos. 6 t o 11). As a class, the former sulfuric acid for comparison, are shown in Table 111. show the effect of prolonged heating by darken' ing in color, while the latter do not darken in the slightest but become viscous and even glassy. A choice from each class may be made Table I. Rejected Group of Colored Substances on the basis of other properties. Table I V Supplier Description SubJtance gives a survey of some other pertinent properIIydrocarbons ties of the most interesting Aroclors and siliS a n t o w n DO Monsanto Chemical Co. Mixture of diphenyl a n d oterphenyl with probably cones compared with sulfuric acid. some n-terphenyl ~~
Santowax 0 -4myldiphenyl Dinonylnaphthalene Halogenated hydrocarbons Aroclor 1262
Monsanto Chemical Co. Eastman Kodali Go. Sharples Chemicals, Inc.
o-Terphenyl
Monsanto Chemical Co.
Chlorinated diphenyl taining 62% chlorine
Alcohols Ceryl alcohol Ethers Di-n-tetradecyi ether
Eastman Kodak Co.
Esters lsoamyl stearate sec-Octyl stearate Tripalmitin Tristearin Diphenyl phthalate Peanut oil Rapeseed oil Nitriles Hexadecyl nitrile
Eastman Kodak Co. Eastman Kodak Co. Eastman Kodak Co. Eastrnan Kodak Co. Monsanto Chemical Co. Eimer a n d Amend Eimer a n d Amend
Arneel 18 D E t h e r alcohols Polyethylene glycol 300 Di-tert-amylphenoxyethanol E t h e r esters Methyl Cellosolve oleate Flexol plasticizer 4 G 0 Di-tert-amylphenoxyet by1 acetate M e t a l organics Tri-p-tert-amylphenyl phosphate Tri-l,2.4-xylenyl phosphate Tri-o-phenylphenyl phosphate ~
con-
Connecticut Hard Rubber Co.
Columbia Organic Chemicals Co. Armour a n d Co. Carbide a n d Carbon Chemicals Corp. Sharples Chemicals, Inc.
Octadecyl nitrile HOCH*(CHnOCHn)zCH,OH, av. mol. wt. 300
Kessler Chemical Co., Inc. Carbide a n d Carbon Polyethyleneglycol di-2Chemicals Corp. ethylhexoate Sharples Chemicals, Inc. Sharples Chemicals, Inc.
~
Sharples Chemicals, Inc. Eastrnan Kodak Co.
e
The data on the Aroclors and silicones were taken mainly from the technical bulletins issued by the manufacturers; the data on sulfuric acid were obtained from secondary reference works. While the .4roclors show no appreciable differences among themselves in heat stability, No. 3 possesses the highest boiling point and the highest flash point; these favorable properties would appear to outweigh its relatively high viscosity and mark it as the best of the Aroclors. Of the silicones, only Nos. 7 and 8 are colorless and remain liquid after prolonged heating at 250' C. There is little t o distinguish between these in boiling point or flash point, but better stability a t 300' C. and l o n w viscosity indicate S o . 7 as superior. Thus Nos. 3 and 7 stand out as the two most satisfactory liquids of those studied. Their heat stability is remarkable when it is realized that paraffin, among the colorless substances, turns yellow within 24 hours when held a t 200" C. Heating for 48 hours a t 200' C., followed by 48 hours at 250" C., the minimum conditions under which these substances exhibit very apparent chemical change, corresponds t o a period of normal use in a melting point apparatus of from several months t o a few years. Both liquids may be heated several times to 300" C. without showing chemi-
ANALYTICAL CHEMISTRY
376
Table 111. Color of Liquids on Prolonged Heating KO.
Substance
Supplier
Sulfuric acid
Description
J. T. Bakpr Chemical
C.P ,
ronrd.
Color a t 2.50' C . 24 hours 48 hours
Color at 200' C.= 24 hours 48 hours Colorless
Colorless
Colorless
Colorless
Color a t 300' C. 24 hours 48 hours
. .......
e
CO.
1
Halogenated hydrocarbons Aroclor 1232
2
.iroclor 1242
3
hroclor 1248
4
Halowax 1012
hlonsanto Cheniiral co. Monsanto Chemical co. Monsanto Chemical Co. Bakelite Corp.
Metal organics Diphenyl o-chlorophenyl phosphate 6 D C 550 fluid 7 Silicone oil 9981LTNV-40 8 Silicone oil 9981LTNV-70 9 Silicone 011 9981LTiXV-70 5
10
11 Oi
ox
Silicone oil 9981LTNV-100 Silicone oil 9981LTNV-ZOO
Temperatures in
327, chlorine
Colorless
Colorless
Yellow tint
Light yellow
Dark yellow
Yellow brown
42% chlorine
Colorless
Colorless
Yellox tint
Light yellou.
Dark yellow
Yellow brown
48% chlorine
Cdorless
Colorless
Yellow tint
Light yellow
Yellow
Yellow brown
rhlorinated naphthalene, aborit aOV0 chlorine
Rrown t i n t
Light brown
... .,
.,. .. ..
.
Yellow tint
Light brown
..... ..
.......
.......
. . . . . .. .
Light yellow Colorless
Light yellow ColorlessC
Light yellow ColorlessC
Light yellowo ColorlessC
Eastinan Kodak Co. Dow Corning Corp. General Electric Co.
,
.....
, .
.I...
A silicone h silicone
Light yellowb Light yellow Colorless Colorlesa
General Electric Co
h silicone
Colorless
Colorless
Colorless
Colorlessc
Colorlessd
General Electric Co.
A
Pale yellow
Pale yellow
Pale yellou
Pale yellow
Pale yellow
General Elertric Co.
silicone, contains oxidation inhibitor A silicone
Colorless
Colorless
ColorlessC
Colorless"
. . . , ...
........
General Electric Co.
.4 silicone
(:olorless
Colorless
Colorless"
,.,
...
.......
C. (corr.)
Table IV.
b
Clear, not turbid.
'
Becomes more viscous.
Physical Properties of Liquids
d
' Colorlesac
Sets t o a gel.
a
,
Incipient boiling with copious fumes.
........ Pale yellowC
.
,
difficulty was experienced with the electrically heated Hershberg apparatus. A slight modification in the length and di..... ameter of the Nichrome wire used in the apparatus has been found desirable in order to attain rapidly the higher temperatures made possible by these liquids. Eight feet of 26-gage (B. and S., 2.550 ohms per foot) wire, wound in two layers of about equal length, are recommended. With this heating element and with No. 3 as the liquid, starting from room temperature a bath temperature of 200' C. can be reached in 4.5 minutes and 300' C. in 10 minutes. Both No. 3 and No. 7 were found unsuitable for use with an internal heating element of bare Nichrome wire; the former turned yellow, while the latter became very viscous.
Coefficient Boiling Specific VjsFlash Fire of ExSpecific NO. Temp.a Gravity cosityb PointC Pointc pansiond Heat H2S04 .. , 1.83e 741 Xone h-one 0.000576 0.340 1 301 1.26-1.27h 48-50 152-154 238 0.000725 , 2 327 1.38-1.39h 80-93 176-180 334 0.000678 ca.0.29i 3 365 1.45-1.46h 185-240 193-196 None 0.000702 ca. 0 . 2 7 i 7 428 0.97i 185 ca. 316 ca. 316 0.001 0.47 8 439 0.97j 325 ca. 316 ca.316 0.001 0.47 ' Temperature of liquid, C. (cox.), when I Calculated from d a t a in other units 0 Cal./g. a t 25-45O C. vi orously boiling. Saybolt Universal seconds a t loODF. h 25' C./25' C. c O c. i Cal./g. a t 20° C. d Co./cc./O c. i 4 t 200 c. 4 250 c./4= c.
'
cal change. No. 3 has been used a t 343" C. (corrected) in the Hershberg apparatus while No. 7 may be heated t o 400' C. (corrected) or a little higher. The choice between K'os. 3 and 7 will depend on the particular type of melting point apparatus used. Prolonged heating causes the former to turn light yellow and the latter to become more viscous; thus, for apparatus without provision for stirring, No. 3 would be preferable. The toxic effects for animals found (3) on prolonged exposure t o Aroclor vapors evolved a t high temperatures would probably rule out the use of No. 3 in apparatus wing an open beaker, but should not be a consideration in apparatus) including the Hershberg apparatus, where there is little or no escape of vapors into the air. The loner flash point of Xo. 3 would also make it less desirable for use in open beakers. Application to Hershberg Apparatus. While both KO. 3 and No. 7 are superior to sulfuric acid in this apparatus, the former has been preferred by the author because of the smaller coefficient of expansion and the smaller energy input requirement. If the product of the specific gravity and specific heat (Table IV) is taken as a measure of the energy required to heat the meltingpoint bath to a given point, it will be seen that KO.3 requires the least energy and sulfuric acid the most. The figures for the products are: No. 3, 0.39; S o . 7, 0.46; and sulfuric acid, 0.62. Othern ise expressed, a given heat input will raise the bath t o a given temperature most rapidly with Xo. 3 and least rapidly with wlfuric acid. Although a liquid of low heat capacity may be disadvantageous, especially in an apparatus heated by a gas flame, because the temperature of t h e bath ic less easily controlled, no
SUMX4RY
Two commercially available liquids, .-iroclor 1248 and silicone oil 9981-LTXV-40, hare been found superior to other compounds tested for use as heat-transfer liquids in apparatus for the determination of melting points. Both substances are colorless, relatively heat-stable, noncorrosive, and nonhygroscopic, and their boiling points are above 350" C. Aroclor 1248 can be used a t temperatures as high as 340' C. and silicone oil 9981-LTNV-40 as high as 400' C. ACKNOWLEDGMENT
The author wishes to express his thanks to the industrial firms listed in the tables, many of n-hich provided samples of their products for this study, and to Ralph F. Wiseman for assistance in the experimental work. LITERATURE CITED (1) Hershbeig, E B., IND. ENG.CHEM.,ASAL.ED.,8 , 312-13 (1936). (2) Maglio, hI. M., Chemist d n a l y s t , 35, 94 (1946). (3) M o n s a n t o Chemical Co.. "The AI odors," Application Data
Bull. P-115. L. M.,A S A I . CHEM. 19, 432 (1947).
(4) W h i t e ,
RECEIVED March 22, 194i.
